1. Field of the Invention
The present invention relates to a Lamb wave device using a Lamb wave propagating in a piezoelectric thin film, and more particularly, to a Lamb wave device used as a resonator or a filter.
2. Description of the Related Art
Heretofore, devices using various waves excited using a piezoelectric effect have been proposed and used. Among these devices, devices using a surface acoustic wave, that is, energy that concentrates in the vicinity of a surface of an elastic body and propagates, have been widely used as a resonator, a filter, and other suitable devices.
On the other hand, according to “Analysis of Lamb Wave Propagation Characteristics In Rotated Y-cut X-propagation LiNbO3 Plate” The Journal of the Institute of Electrical Communication Engineers of Japan, 85/5, Vol. J68-A, No. 5, pp. 496-503, a device using a Lamb wave propagating in an elastic body, which is different from a Rayleigh wave, is disclosed. The Lamb wave is one type of bulk wave, that is, one type of plate wave, and is an elastic wave which propagates in a piezoelectric body while reflecting off two primary surfaces thereof when the thickness of the piezoelectric body is equal to or less than the wavelength of the elastic wave. Instead of the Lamb wave, an SH wave is also known. According to “Handbook of Elastic Wave Device Technology” (published in 1991 by Ohm Co., Ltd.), “the mode change of an SV wave and that of a longitudinal wave (compressional wave) occur at two surfaces of a plate, and by complicated coupling therebetween, a plate wave called a Lamb wave is generated.”
As disclosed in “Analysis of Lamb Wave Propagation Characteristics In Rotated Y-cut X-propagation LiNbO3 Plate”, The Journal of the Institute of Electrical Communication Engineers of Japan, 85/5, Vol. J68-A, No. 5, pp. 496-503, since the Lamb wave, which is an elastic wave, propagates in a plate-shaped piezoelectric body while reflecting off two surfaces of the plate-shaped elastic body, the properties of the Lamb wave are different from those of the Rayleigh wave. In the Lamb wave, since the velocity dispersion properties are present, two surfaces can be used, and thus, the Lamb wave is likely to obtain a larger electromechanical coupling coefficient than that of the Rayleigh wave.
In “Analysis of Lamb Wave Propagation Characteristics In Rotated Y-cut X-propagation LiNbO3 Plate” The Journal of the Institute of Electrical Communication Engineers of Japan, 85/5, Vol. J68-A, No. 5, pp. 496-503, a Lamb wave device using a piezoelectric thin film of 90°-rotated Y-X LiNbO3 is disclosed. In this document, the A1 mode, which is an asymmetric mode having a high acoustic velocity and a large electromechanical coupling coefficient, is excited.
According to the above Lamb wave device disclosed in the “Analysis of Lamb Wave Propagation Characteristics In Rotated Y-cut X-propagation LiNbO3 Plate”, The Journal of the Institute of Electrical Communication Engineers of Japan, 85/5, Vol. J68-A, No. 5, pp. 496-503, since the Lamb wave is used in a structure in which an IDT electrode is provided on a piezoelectric thin film, the electromechanical coupling coefficient is likely to be increased. However, when a Lamb wave device as disclosed in “Analysis of Lamb Wave Propagation Characteristics In Rotated Y-cut X-propagation LiNbO3 Plate”, The Journal of the Institute of Electrical Communication Engineers of Japan, 85/5, Vol. J68-A, No. 5, pp. 496-503 is made, although a pass band can be obtained in a high frequency region, spurious responses are likely to be generated in the pass band or the attenuation band.